Remember that you are dust and to dust you shall return.

With that somber phrase from the Episcopal Book of Common Prayer, or with slight variations in other churches, Christians the world around tomorrow will mark Ash Wednesday, the beginning of 40 days of Lent.

In Roman Catholic, Episcopal, Lutheran and some individual churches of other traditions, the faithful will receive on their foreheads the imprint of ashes, traditionally made from palm fronds left from the previous year's Palm Sunday services. In other churches, the day will be marked by a variety of worship services.

Ashes, which have been part of Ash Wednesday services since the 11th century, are used in the rites of many religions. But in the Old Testament, they signified mortality, sorrow and repentance.

For many Christians, Lent has traditionally been a time of self-denial--fasting from certain foods, giving up cigarettes or sweets, abstaining from certain entertainments such as dancing or movies.

But in recent years there has been a growing emphasis on a positive approach to Lent and in preparing for Easter by joining study groups, going on a retreat, reading spiritual books. At Easter, Christians celebrate Christ's resurrection from the dead and his promise that those who follow him will have eternal life. It is the heart of the Christian Gospel.

Roman Catholics today no longer follow the austere rules of fasting and abstinence for the 40 days of Lent that were enforced a generation ago. Church rules today call upon all who are between the ages of 21 and 59 to abstain from eating meat on each Friday of Lent and to observe Ash Wednesday and Good Friday as fast days in which they eat only one full, meatless, meal.

Locally, numerous congregations of varying denominations have planned Lenten study groups and lecture sessions for their members.

One of the more ambitious ventures on a community-wide basis will take place at the Washington Cathedral on Saturday. The day-long program, which begins at 10 a.m., is designed to introduce participants to the variety of spiritual resources available through churches and other religious institutions in the Washington area.

"God is so great that we can always learn something more about him by listening to the spiritual experience of people from differing backgrounds," said Canon Michael Hamilton of the cathedral staff, who is planning the day around the theme, "To Refresh the Spirit."

Catholic and Protestant leaders will conduct morning and afternoon workshops and Bible study groups on such themes as spiritual healing, techniques of prayer, the church and peace and facing death and resurrection.